Attenuation Flashcards
attenuation
the weaking of the sound beam as it travels
clinical importance of attenuation (3)
limits our imaging depth
must be compensated for (TGC/gain)
can be useful in diagnosis
bel/decibel are what
relative units of measurement expressing the loudness (intensity level) of sound waves
1 bel (B) = ____ decibels (dB)
10
0 dB as a percentage for US
100%
instrument ____ uses decibels to determine the ____ of the sound leaving the transducer
output
power
dynamic range used decibels to express the ______ of shades of grey displayed on a monitor
number
gain and TGC use decibels to express the amount of ______ or ______ required to optimize the returning echoes
amplification
attenuation
formula to calculated bels/dB
b=log(new I / original I)
dB=10log (new I / original I)
power formula
dB=10log(new P / original P)
voltage formula
dB=20log(new V / old V)
1/2 original intensity = ___ dB drop
3
.1 of og intensity= ___ dB drop
10
10db drop =
.1 of og intensity
3dB drop=
1/2 original intensity
input/output power = what in power formula
input= Po (original P)
output= Pn (new P)
attenuation coefficient
amount of attenuation that occurs with each one cm travelled
attenuation coefficient in soft tissue
.5 dB for every one cm travelled per 1 MHz
attenuation coefficient in soft tissue is equivalent to _____
1/2 freqeuncy
Total attenuation formula
Tatt= att. coef. x path length (cm)
total attenuation formula for soft tissure
Tatt= 1/2f x path length (cm)
half-value layer is what
the distance sound must travel in a material to reduce the intensity to half its original value
attenuation can vary with the following (3)
nature of tissue (dead/alive)
frequency of the US
depth
half-value layer formula
Tatt=1/2f x path
but
3dB loss= 1/2 og intensity
so
formula=
3dB=1/2f x path
5 reasons for attenuation
absorption
reflection
refraction
scatter (kinda a part of reflection)
wave-front divergence
absorption (what, special, accounts for)
the conversion of sound energy to heat and is the dominant factor in attenuation (accounts for about 80%)
3 things affecting absorption and what is dominant
viscosity
relaxation time on molecules
frequency (dominant)
what is viscosity
thickness - ability of molecules to slide past one another
increased viscosity = _____ resistance, ______ friction, and thus, _____ attenuation
greater
more
increased
what is friction (and what part of attenuation it accounts for)
how sound energy is converted to heat
absorption
relaxation time (and what part of attenuation it accounts for)
time it takes vibrating molecules to come to rest
absorption
if molecules cant come to rest before the next compression phase, than ___ energy is require to ____ its direction. This produces ____
more
reverse
heat
increase relaxation time= ____ absorption
increased
how does frequency affect absorption
F increased = less time available for molecules to recover during relaxation process = more absorption
two types of reflection
specular
non-specular
specular reflection occurs when
a sound beam hits a large, smooth surface
what is a large surface relative to
to the frequency used and is greater than one wavelength in diameter
good specular reflector examples
diaphragm, portal vein
intensity of sound reflected relies on two things
angle of incidence
acoustic impedance of two media
angle of incidence equals the angle of
relfection
reflections that have non-perpendicular incidence may …
not return to probe
________ incidence helps improve relfection
perpendicular
acoustic impedance AKA/what (formula)
character impedance
describes the relationship between acoustic pressure and the speed of particle vibrations in a sound wave
Impedance (Z) = density (p) x velocity (c)
how fast a sound moves through a medium depends on mediums ______ (propagation speed)
density and stiffness
impedance formula
impedance (Z) = density (p) x velocity (c)
unit of impedance
Rayls (Z)
Z increases with ______ density or velocity
increases
acoustic impedance ____ depend on frequency
does not
the _____ the Z value difference at the interface of 2 media the bigger the reflection
larger
Rayls (Z) = unit for impedance
if Z values are the same =
no relfection
Rayls (Z) = unit for impedance
intensity reflection coefficient
amount of sound that reflects at an interface
intensity reflection coefficient formula
IRC = reflected intensity (Ir) / incident intensity (Ii)
or
IRC = (Z2 - Z1) / (Z2 + Z1) all squared
IRC answers will always be
less than one (multiply by 100 for percentage)
intensity transmission coefficient formula
ITC = 1 - IRC
what is the intensity transmission coefficient
amount of sound transmitted
more sound reflected = _____ sound transmitted
less
reflection occurs because of a difference in ____values across an interface
Z (Rayls)
it ___ possible for 2 tissues to have different sound velocities but the same Z values
is
reflection vs refraction
sound returning to the probe from an interface
vs
sound that is transmitted across the interface
Snell’s law of optics
Sin θi / Sin θt = Vi / Vt
V=velocity
i= incidence
t= transmitted
sound can bend across an interface due to a difference in media ____
veolcity
velocities across interface are equal =
no refraction
velocity of first medium is greater than that of second (V1>V2) =
sound bends towards normal (new angle less than what you started with)
velocity of of first medium is less than the second (V1<V2) =
sound bends away from normal (larger angle than what you started with)
total internal reflection
occurs when V1 is less than V2 and the angle reaches a critical value where it is straight with the interface
refraction _____ relate to Z values
does not
scatter is the result of
sound interacting with interfaces smaller than a wavelength and rough
scatter is typically from ____ media, and is _____ of sound direction
heterogenous
independent
scatter is responsible for the internal _____ of organs
texture
scatter results in the incident sound beam ____
breaking up into many different echoes each with its own intensities and direction
amount of scatter depends on (2)
frequency: higher F = more scatter
reflector size: smaller = more scatter
acoustic speckle
phenomenon where the interference patterns from echoes undergo multi-path scattering resulting in potential for brightness non-uniformities
phenomenon where the interference patterns from echoes undergo multi-path scattering resulting in potential for brightness non-uniformities
acoustic speckle
Rayleigh scatter
specific to RBC’s
RBC’s are way smaller than the wavelength of sound = very weak scatter = we don’t see blood flow in vessels at faster velocities (can sometimes see in IVC where slower/clumps near heart)
as the sound beam diverges, the intensity of the beam _____ and adds to the overall ____ of the beam
weakens
attenuation
Media that measure less than one wavelength and contribute to the parenchyma or fine texture pattern of an image
non specular reflectors
The dominant factor in attenuation in which the sound energy is converted to heat
absorption
The redirection of sound in many directions and intensities by rough surfaces or heterogeneous media measuring less than one wavelength
scatter
The relative unit of measurement that expresses the intensity or loudness of sound
bel
The throwing back of sound without absorbing it as the result of a difference in acoustic impedances across an interface
reflection
One tenth of a Bel
decibel
large smooth surfaces, measuring greater than one wavelength, that return relatively large amounts of sound to the source
specular reflectors
The weakening of sound as it travels
attenuation
The ability of the molecules of a medium to slide past one another
viscosity
The amount of time it takes for the molecules in a medium to come to rest after a mechanical force is applied
relaxation time
The bending of sound across an interface when the angle of incidence is non-perpendicular and the velocities of sound differ in the two media
refraction
The amount of tissue required to attenuate sound to half of its original intensity
half value layer
The amount decibels lost per centimeter of travel
attenuation coefficient
The spreading out of the sound beam as it travels
divergence
What type of reflectors will result in scatter?
A) Specular reflectors
B) Non-specular reflectors
C) Reflectors that are smooth
D) Reflectors that are greater than 1 wavelength
b
What happens to the amount of absorption if there is an increase in relaxation time?
A) It increases
B) It decreases
C) It stays the same
D) Relaxation time does not effect absorption
a
What term describes an image that has non-uniform areas of brightness as the result of multipath echoes interfering when they reach the probe?
A) Refraction
B) Acoustic speckle
C) Destructive interference
D) Constructive interference
b
If the frequency of the sound emitted is increased then what is true of the total attenuation under the same scanning conditions?
A) It increases
B) It decreases
C) It is unchanged
D) Frequency has no impact on total attenuation
a
Which of the following conditions may result in total internal reflection of sound at an interface?
A) Velocity 2 is less than velocity 1 with perpendicular incidence
B) Velocity 2 is greater than velocity 1 with perpendicular incidence
C) Velocity 2 is less than velocity 1 with non-perpendicular incidence
D) Velocity 2 is greater than velocity 1 with non-perpendicular incidence
d
Sound has an intensity of 30 dB and is attenuated to 27 dB after traveling 3 cm in soft tissue. What is true of this distance?
A) It is the half value layer
B) It reduced the power by half
C) It reduced the intensity by half
D) All of the above are correct
d
How many decibels per centimeter are attenuated for every 1 MHz of frequency in soft tissue?
A) 0.3 dB
B) 0.5 dB
C) 0.7 dB
D) 1.0 dB
b
What is the most dominant factor in attenuation?
A) Scatter
B) Reflection
C) Refraction
D) Absorption
d
Which of the following is needed for reflection of sound to occur at an interface?
A) Z values must differ
B) Densities must differ
C) Velocities must differ
D) Z values must be equal
a
name all things affected when increasing frequency emitted from probe
Decrease: wavelength, T, PD, SPL, DF
increase: res, absorption. scatter
tools/ scanning procedures that can compensate for attenuation (and what problems it helps
decrease F (absorption, scatter)
get perpendicular (reflection, refraction, scatter)
adjust divergence (wave front divergence)
increase power,
change window
use gel
TGC/Gain
what values relates the spatial peak intensity to the spatial average intensity and how
BUR= SP/SA
what factor has the greatest influence on the velocity of sound
compressibility
what type of reflector is the parenchyma of the body/why
non-specular/scatter as smaller than a wavelength
what is the reduced attenuation of tissues seen deep to anechoic structures called
posterior/acoustic enhancement
through transmission
how does Huygen’s principle affect transducers sensitibity
con/destructive interference from wavelets give natural focus, manipulate focus = changes sensitivity
what is it called when two waves are neither completely in or out of phase
beat frequency
When scanning from tissue 1, with an acoustic velocity of 1550 m/s, to tissue 2 that has an acoustic velocity of 1530 m/s, there is no reflection at the interface. How could this be possible?
Reflection depends on Z values and not velocities. It is possible that the tissues have equal Z values and different velocities.
